Devices and methods for sampling and analysis of airborne particles (aerosols) from a moving airstream find use in environmental and industrial sciences and for surveillance. The most informative aerosol particles for the purposes contemplated here are those less than 20 microns in apparent aerodynamic diameter (AD), and particularly those particles of about or less than 10 microns AD, because these particles remain suspended in air for longer periods of time and more readily penetrate and lodge in the respiratory tract.
Aerosol sampling is made more difficult by wind, mist, dust and rain, and can be complicated when the aerosol sampling device is mounted on a mobile vehicle such as a truck, airplane, marine vessel or riverine vehicle. Design of a sampling inlet can also be problematic because of the accumulation of sand, salt crystals, dust, and fibers, and also water spray. Under heavy loading, particulate solids such as dust and fibers have been known to accrete so as to block the sampling inlet or reduce sampling efficiency and performance. Filter pads sometimes used for trapping aerosols also become blocked or tear when wetted by rain or mist.
Improvements are needed in the art to develop a sampling inlet that is resistant to weather and effectively samples the most informationally-rich particles from moving airstreams, i.e., those less than 20 microns AD. Because the direction of flow of the moving airstream can shift, an omnidirectional sampling inlet is needed. Such a device must be effective when mounted in a moving vehicle or stationary while subjected to increased surface winds, for example.
Ideally, the sampling inlet has a high efficiency in collecting particles less than 10 microns AD and a high efficiency in excluding particles greater than 20 microns AD under a range of ambient conditions, while not affected by changes in apparent windspeed or by rain. Advantageously, the device is also resistant to fouling by dust or fibers and is self-cleaning.
Wedding, in U.S. Pat. No. 4,461,183, describes an omni-directional aerosol sampler with cylindrical external housing (1), internal skirted flow deflector (2), and cyclonic particle fractionators, ie. having airfoil-shaped vanes for forming a cyclonic flow in the downtube. As shown in FIG. 1, a flow of air through the device is pulled by a downstream vacuum and is made cyclonic by airfoil-shaped vanes (3) at the inlet (4) to the particle fractionator. The particle fractionator consists of a vertical, blind-bottomed, tubular trap (5) for receiving the downwardly directed vortex through a narrow downtube (6) mounted centrally under the vanes (3) and extending into the trap (5), such that air entering the trap must reverse course and rise to exit through an annular outlet (7) at the top of the fractionator. The device is configured so that half the particles greater than 10 microns impact the walls of the particle fractionator and are collected in the trap (5). The aerosol is thus classified into a coarse fraction, which is trapped in the device, and a fine fraction that exits the device and may be captured on a filter cassette. The airfoil-shaped vanes are described as a two-dimensional shape (Col 4, lines 42-47) that is effective in eliminating particle deposition or build up. A lower flared portion of the skirt (8) directs air upwardly into the vanes (3). The curvature and diameters of the intake duct (9) are selected so as to prevent turbulence or deposition of particles upstream from the vane assembly and trap (Col 5, lines 1-3). Thus particle removal occurs in and on the walls of the trap (5), which functions as a cyclonic impactor. This design is inherently not self-cleaning and cannot continue to function without period emptying of the trap. Condensate collecting in the trap cannot be drained without disassembly. It is thought that continued buildup of particles in the intake duct (9) will interfere with operation of the device. Relatively little consideration is given to the flow conditions around the bottom of the skirt so as to avoid external pressure ridges that would act to deflect and exclude sampling of smaller particles.
Cyclonic impactors are associated with increased inelastic impactor collisions unless used in conjunction with wetted wall devices and thus tend to take in an excess of larger particles that escape impaction by bouncing off the impactor surface.
The device also includes an internal bug screen. The device of FIG. 1B is reportedly scaled to draw 113 to 1133 liters/min (4 to 40 cfm) and to operate in windspeeds up to 24 kilometers per hour (15 miles/hr). At higher windspeeds, collection of particles in the desired size range may be impeded because of impaction in the intake manifold, which unfortunately functions as an inertial or bluff body impactor. Also, the trap is not readily cleaned and may clog or fill with condensate, fibers and dust during operation under adverse conditions.
Interestingly, the preferred system currently in use is not the Wedding device but rather the device (20) of FIG. 2, which is adapted from compressor intakes with noise-suppression, and is used by the US Department of Defense in their Dry Filter Unit 2000 particle capture technology. This unit is used in conjunction with a standardized duplex filter cassette which is removed periodically for immunoanalysis. Particles entering the updraft tubes (21) are classified by elutriation, and the particle depleted fraction is then directed to the vacuum exhaust. Elutriation is known to have a relatively poor particle size resolution capability and a broad cutsize limit. We have found that the units have relatively low efficiency in capture of informationally-rich particles with increasing wind speed. The unit is not self-cleaning and particles frequently accumulate in the housing (22) between the updraft tubes (21). Also, moisture that enters the filter cassette can lead to false negatives. The system would benefit from increased sensitivity, improved cutsize resolution, and better wind-resistance.
One improved system is shown in FIG. 3. This system uses an eductor (31) with inertial impactor (32) to separate coarse material from a bending flow stream that is then directed onto a duplex filter cassette (33). The duplex filter cassette is the same as the one used with the Dry Filter Unit 2000 developed by the Department of Defense. The inlet units (30) are designed to be stacked for storage and feature a hinged housing for easy access to the filter cassette. Unfortunately, this prior art device is relatively inefficient at classifying particles by size due to its geometry and the close proximity of the inertial impactor and sampling cassette, which may allow entry of raindrops and is associated with accumulation of condensation inside the ductwork due to lack of drainage. Also at issue is the higher pressure drop, power loss and potential for fouling because inlet flow is passed through a small constriction.
As is also known in the art, downstream virtual impactors, cascading virtual impactors, bluff-body impactors, liquid impingers, filters, and the like may be used to further concentrate, classify or capture aerosol species by size or composition. While these techniques are well known, the design of the initial sampling inlet remains problematic because of instabilities of the outside air mass relative to the intake and because of lack of streamlining of the outside housing of the intake. Also of concern, the inlet may not be self-purging of coarse particles and fibers, or of water vapor or rain.
In U.S. Pat. No. 6,530,287 to Rodgers, a rain shroud covers canted intake nozzles spaced around a central chamber; the intake nozzles are canted so as to drain moisture that enters under the shroud while admitting particles. This unit lacks capacity to fractionate particles by size and is generally non-specific in admitting large and small particles into the central chamber.
Accordingly there remains a need in the art for improved devices and methods for omnidirectional collection of aerosols that overcome the above disadvantages.